A MnIII2FeII chain with a [–MnIII–(OPh)2–MnIII–ON–FeII–NO–] repeat unit was synthesized via the assembling reaction of MnIII salen-type dimers and FeII pyridyloximate complexes, where the –(OPh)– and –ON– bridges represent a biphenolate bridge and an oximate bridge, respectively. The bulky counter anions, BPh4−, which surround the chain to form a zeolite-like brick wall, successfully isolate the chains from a magnetic point of view. This compound is isostructural with Mn2Ni–BPh4, which is an SCM with ST = 3 (H. Miyasaka, A. Saitoh, M. Yamashita and R. Clérac, Dalton Trans., 2008, 2422). Because the FeII unit is diamagnetic, the MnIII dimer, which has the potential to be a single-molecule magnet (SMM), is nearly magnetically isolated, although a weak antiferromagnetic interaction with J/kB ≈ −0.1 K is perturbed between the MnIII dimers with ST = 4 via the FeII unit. Ac susceptibility data shows that two types of relaxation of the magnetization are present, which are attributed to SMM and chain relaxations.